Alunite is a potassium aluminum sulfate mineral having the general formula: KAl.sub.3 (OH).sub.6 (S0.sub.4).sub.2. Alunite ores, also typically contain varying amounts of sodium-containing minerals and/or silica, SiO.sub.2 Several processes have been developed for recovering aluminum values from alunite ore, many of which also include recovery of the potassium values as K.sub.2 SO.sub.4. All have been plagued by some major economic flaw; for example, expensive purchased reagents, complicated and capital-intensive processes, requirements for sulfuric acid production to handle sulfur dioxide off-gas, effluent pollution problems, and/or high energy requirements for thermal pretreatment of the ore.
Alunite is not very soluble in water and, as such, many mineral recovery processes involve caustic leaching of the ore to solubilize the potassium and aluminum values into the leach liquor from which they are subsequently separated and recovered. U.S. Pat. Nos. 3,983,211; 3,890,425; 3,890,426; 4,029,737; 4,064,217 and 4,057,611 exemplify prior art teachings with respect to caustic leaching with NH.sub.4 OH or NaOH, KOH and mixtures thereof. Many prior art processes such as those of U.S. Pat. Nos. 3,890,425; 3,890,426; 4,029,737 and 4,057,611 require roasting or dehydrating the alunite prior to leaching. In each of these prior art processes, the potassium values of the alunite ore are extracted in the initial leaching consistent with what has heretofore been considered the only effective method of mineral value recovery. In such methods, the alumina-containing residue is treated in a typical Bayer-type circuit for recovery of high grade alumina, i.e. leached into solution typically a caustic solution of NaOH and a mixture of NaOH and KOH, and reprecipitated.
It is known that where significant amounts of silica are present in the ore, losses of aluminum values by precipitation of insoluble alkali aluminosilicates can occur. U.S. Pat. Nos. 3,983,211 and 3,984,521 teach leaching processes wherein production of aluminosilicates is minimized. In both patents, a mixture of KOH and NaOH is used as the initial leach. In addition, U.S. Pat. No. 3,984,521 teaches that the initial leach must be carried out at a temperature below 60.degree. C. with the disadvantage of slow kinetics. U.S. Pat. No. 3,983,211 permits a higher temperature leach but requires a substantial excess of sodium ions relative to potassium ions. In U.S. Pat. No. 3,983,211 the aluminum and potassium ore values are both extracted into the initial leach liquor. U.S. Pat. No. 3,984,521 teaches solubilizing the aluminum values in the relatively low temperature leach while leaving potassium and sodium sulfates and silicates in the leach residue, but because of the substantial amount of sodium present requires a separation between the sodium and potassium sulfates which are subsequently extracted together from the residue.
French Pat. No. 791,021 teaches a process for leaching alunite with a KOH leach and solubilizing the aluminum, potassium, and sulfur values from the ore into the leachate. Potassium, sulfur and silicate values are crystallized from the leachate by cooling, with subsequent processing of the leachate to recover aluminum values. The process of French Pat. No. 791,021 is directed primarily toward production of pure alumina and does not teach any overall system demonstrating recovery of pure K.sub.2 SO.sub.4 or regeneration of KOH by any of the methods utilized herein.
Other references to potassium hydroxide leaching of alunite include G. Hohorst et al., J. Kim Enge. (China), 4, 21-8 (1937); Chemical Abstracts-6828E; Japanese Pat. No. 76-20, 438 (1973); Chemical Abstracts, 86, 191990F, and I. Gruncharov, Chemical Abstracts 98-218029M (Bulgarian).
None of this prior art discloses or suggests the separation of aluminum values into a primary leach liquor by using a potassium hydroxide leach saturated with K.sub.2 SO.sub.4 and thereby leaving the potassium and sulfur values in the primary leach residue.
J. Gullichsen et al., "Recovery of Sodium-Base Pulping Chemicals by Bicarbonation and Crystallization," Tappi, Vol. 51, No. 9, 395-400 (Sept. 1968) and U.S. Pat. No. 3,134,639 disclose sulfidization and carbonization reactions for converting alkali metal sulfates to carbonates, but this art does not show subsequent conversion to the hydroxide nor recycle of the hydroxide solution to a leach process for alunite. Similarly, German Pat. No. 590158 (1933) shows conversion of potassium sulfate to potassium formate with subsequent conversion to potassium carbonate, but this art does not show or suggest subsequent conversion to the hydroxide, nor recycle of the hydroxide to an alunite leach process.
The pyrohydrolysis of sodium sulfate to sodium carbonate in a green processing with coal and water has been described in E. Horntvedt, "SCA-Billerud Recovery Process Goes On-Stream," Pulp and Paper International, August, 1968. Pyrolysis of potassium sulfate with coal or reducing gases has also been described in E. J. Lahoda et al., "Engineering Design for the Westinghouse MHD Seed Regeneration Process," 7th International Conference on MHD Electrical Power Generation, Vol. 1, Page 351; J. I. Joubert et al., "Kinetics of Regeneration of Spent Seed from MHD Power Generation Systems," U.S. Energy Research and Development Administration, Pittsburgh Energy Research Center, Energy Conversion, Pittsburgh, Pennsylvania; and "Study of Seed Reprocessing Systems for Coal Fired Open Cycle Coal Fired MHD Power Plants, Task I, Selection of Processes for More Detailed Study," D.0.E. Contract No. DE-AC 02-79ET 15613, July 17, 1980. These authors all require a two-stage reaction in which potassium sulfate is converted to potassium sulfide at high temperature, followed by low temperature oxidation of the potassium sulfide to potassium carbonate, and none teaches or suggests the parameters or viability of a process for the direct, one-stage pyrolysis conversion of an alkali metal sulfate to the corresponding carbonate. In addition, none teach or suggest further conversion of the carbonate to the hydroxide.
None of the above-described art shows or suggests the desirability of using alkali metal and sulfur values contained in an ore to generate a caustic leach solution for that ore.